Method and apparatus for mitigating power-control error during a soft handoff in a wireless communication system

ABSTRACT

A plurality of base stations ( 600 ) are programmed ( 702 ) with a uniform power-control bit pattern to be sent to a mobile unit during a plurality of power-control bit times, before the mobile unit is acquired on a reverse link. The plurality of base stations are further arranged and programmed to synchronize ( 704 ) transmissions of the uniform power-control bit pattern such that the plurality of base stations, when transmitting, send identical power-control bits during each of the plurality of power-control bit times.

FIELD OF THE INVENTION

This invention relates in general to wireless communication systems, andmore specifically to a method and apparatus for mitigating power-controlerrors during a soft handoff in a wireless communication system.

BACKGROUND OF THE INVENTION

Prior-art wireless communication systems have employed a special“slow-power-up” power control technique to prevent power spikes on thereverse channel from the mobile unit to the base station during theinitiation of a call and during a handoff. Recently, “soft” handoffshave become increasingly popular, because of the advantages they canprovide. In a soft handoff, the mobile unit initially monitors more thanone base station simultaneously in order to facilitate a handoff to thebase station that can provide the best signal quality.

Unfortunately, monitoring more than one prior-art base station canresult in conflicting power-control information being sent to the mobileunit, sometimes causing the mobile unit to decrease power when, in fact,all the monitored base stations are signaling the mobile unit toincrease power!

Thus, what is needed is a method and apparatus for mitigatingpower-control errors during a soft handoff in a wireless communicationsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages inaccordance with the present invention.

FIG. 1 is an electrical block diagram of a prior-art wirelesscommunication system.

FIG. 2 is a diagram depicting an exemplary power-control bit sequence.

FIG. 3 is a diagram depicting power-control errors that can occur duringsoft handoff in the prior-art wireless communication system.

FIG. 4 is a diagram depicting a technique for mitigating power-controlerrors during soft handoff in accordance with a first embodiment of thepresent invention.

FIG. 5 is a diagram depicting a technique for mitigating power-controlerrors during soft handoff in accordance with a second embodiment of thepresent invention.

FIG. 6 is an electrical block diagram of an exemplary base station inaccordance with the present invention.

FIG. 7 is a flow diagram of an exemplary method in accordance with thepresent invention.

FIG. 8 is an electrical block diagram of an exemplary communicationsystem infrastructure in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In overview, the present disclosure concerns wireless communicationsystems that provide service for communications units or morespecifically a user thereof operating therein. More particularly,various inventive concepts and principles embodied as a method andapparatus for mitigating power-control errors during a soft handoff in awireless communication system for use in equipment with suchcommunications systems will be discussed and disclosed. Thecommunications systems of particular interest are those being deployedand developed such as CDMA (Code Division Multiple Access), W-CDMA(Wideband-CDMA), CDMA2000, 2.5G (Generation), 3G, UMTS (Universal MobileTelecommunications Services) systems and evolutions thereof that utilizespread spectrum signals, although the concepts and principles haveapplication in other systems and devices as well.

The instant disclosure is provided to further explain in an enablingfashion the best modes of making and using various embodiments inaccordance with the present invention. The disclosure is further offeredto enhance an understanding and appreciation for the inventiveprinciples and advantages thereof, rather than to limit in any mannerthe invention. The invention is defined solely by the appended claimsincluding any amendments made during the pendency of this applicationand all equivalents of those claims as issued.

It is further understood that the use of relational terms, if any, suchas first and second, top and bottom, and the like are used solely todistinguish one from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions.

Much of the inventive functionality and many of the inventive principlesare best implemented with or in one or more conventional processors, orwith integrated circuits (ICs) such as custom or application specificICs. It is expected that one of ordinary skill, notwithstanding possiblysignificant effort and many design choices motivated by, for example,available time, current technology, and economic considerations, whenguided by the concepts and principles disclosed herein will be readilycapable of programming such processors, or generating such ICs withminimal experimentation. Therefore, in the interest of brevity andminimization of any risk of obscuring the principles and conceptsaccording to the present invention, further discussion of suchprocessors and ICs, if any, will be limited to the essentials withrespect to the principles and concepts employed by the preferredembodiments.

Referring to FIG. 1, an electrical block diagram of a prior-art wirelesscommunication system 100 comprises a wireless communication systeminfrastructure 108, coupled wirelessly to a plurality of wirelesssubscriber units 102, also herein referred to as “mobile units” 102. Thewireless communication system infrastructure 108 comprises a pluralityof base stations 104 coupled to a central controller/switch 106 forcontrolling the infrastructure 108 and for switching calls within theinfrastructure 108 and between the infrastructure 108 and a network 112,such as the public switched telephone network. The communication system100 including the wireless subscriber or communication units is aconventional system such as a CDMA system with corresponding subscriberhandsets.

Referring to FIG. 2, a diagram depicts an exemplary power-control bitsequence 200 that can be utilized to control the power of a mobile unitin a wireless communication system, such as a code division multipleaccess (CDMA) system, or any other system that can benefit from mobilepower control. The horizontal axis represents time. The power-controlbit sequence 200 preferably is sent on a forward link by one or morebase stations 104 to the mobile unit 102 in response to an originationrequest received from the mobile unit 102. In the prior-art system 100,the power-control bit sequence 200 is usually a “slow-power-up”sequence, which includes mostly alternating “power-up” bits (representedin FIG. 2 by up arrows) and “power-down” bits (represented by downarrows), with an occasional extra power-up bit. The exemplarypower-control bit sequence 200 comprises two pairs 202, 204 ofconsecutive power-up bits, with the remainder of the bits alternating,for a total of nine power-up commands and seven power-down commands perframe of sixteen power-control bits. Such a sequence is intended togradually increase the power of the mobile unit 102 to prevent a power“spike” from occurring before the base station 104 has acquired themobile unit 102 on a reverse link and established a closed-loop powercontrol.

It is worth noting that, in the prior-art system 100, the exactstructure of the power-control bit sequence can be selected by thesystem operator. For example, one system operator may choose to use apower-control bit sequence exactly like the exemplary power-control bitsequence 200, while another system operator may choose to move the twopairs 202, 204 of consecutive power-up bits to a different location inthe frame, while yet another system operator may choose a fasterpower-up sequence that uses two pairs of three consecutive power-upbits, and so on. Further complicating the issue, some systems use apower-control bit sequence that is longer than one frame, and thus doesnot repeat in every frame. These differences can cause operationaldifficulties during soft handoff, as will be explained next.

Referring to FIG. 3, a diagram 300 depicting power-control errors thatcan occur during soft handoff in the prior-art wireless communicationsystem 100 comprises a first power-control bit sequence 310 includingtwo pairs 316, 318 of consecutive power-up bits sent by a first basestation attempting to communicate with the mobile unit 102. The diagram300 further comprises a second power-control bit sequence 312 includingtwo pairs 302, 304 of consecutive power-up bits sent by a second basestation attempting to communicate with the mobile unit 102. Note thatthe two pairs 316, 318 are offset in time from the two pairs 302, 304.The power-control bit sequence 314 depicts how the mobile unit 102 willresolve the first and second power-control bit sequences 310, 312. Whenthe mobile unit 102 simultaneously receives two conflictingpower-control bits (one up and one down), the protocol rules typicallyrequire the mobile unit to resolve the conflicting bits as a power-downbit. The power-control bit sequence 314 resolved by the mobile unit 102thus includes two pairs 306, 308 of consecutive power-down bits. Themobile unit 102 disadvantageously will gradually reduce its transmittedpower, even though both base stations 104 are requesting the mobile unit102 to gradually increase its transmitted power! The end result of thisunfortunate behavior is that the base station is unlikely to everacquire the mobile station on the reverse link, resulting in a systemfailure and at some point a disappointed user.

Referring to FIG. 4, a diagram 400 depicting a technique for mitigatingpower-control errors during soft handoff in accordance with a firstembodiment of the present invention comprises first and second uniformpower-control bit sequences 402, 404 sent from first and second basestations during a plurality of power-control bit times (represented bythe position of the arrows along the horizontal axis). Note that thefirst and second uniform power-control bit sequences 402, 404 areidentical to one another. The transmissions of the first and seconduniform power-control bit sequences 402, 404 are timed such that thefirst and second base stations send identical power-control bits duringeach of the plurality of power-control bit times. Note that first andsecond pairs 408, 410 of consecutive power-up bits of the first uniformpower-control bit sequence 402 occur simultaneously with the first andsecond pairs 412, 414 of consecutive power-up bits of the second uniformpower-control bit sequence 404. Note further that the power-control bitsequence 406 resolved by the mobile unit 102 correctly includes twopairs 416, 418 of consecutive power-up bits. Thus the mobile unit 102advantageously and appropriately controls the output power level inaccordance with the power-control bit sequence provided by each basestation, and acquisition of the mobile unit is likely to occur.

Referring to FIG. 5, a diagram 500 depicting a technique for mitigatingpower-control errors during soft handoff in accordance with a secondembodiment of the present invention comprises a first power-control bitsequence 502 transmitted by a first base station, and a secondpower-control bit sequence 504 transmitted by a second base station.Note that the bit pattern repeats every frame. Thus, it is possible forthe first base station to start its transmission of the firstpower-control sequence 502 at the start of the first frame, and for thesecond base station to begin its transmission of the secondpower-control bit sequence 504 at the start 508 of the second frame.Note that the power-control bit sequence 506, as resolved by the mobileunit, is correct and is unaffected by the late start of the secondpower-control bit sequence 504.

It will be appreciated that, in yet another embodiment, a power-controlbit sequence can require more than a single transmission frame of theforward link from the base station to the mobile unit before the patternrepeats. In that embodiment, timing the transmissions comprisessynchronizing the transmissions from each of the base stationsattempting to communicate with the mobile unit such that thetransmissions start at substantially identical times.

Referring to FIG. 6, an electrical block diagram of an exemplary basestation 600 in accordance with the present invention comprises aconventional processor 604, e.g., a processor of the MC68000 familyavailable from Motorola, Inc., for controlling the base station 600, anda conventional wireless transceiver 602, such as the wirelesstransceiver employed in CDMA base stations available from Motorola, Inc.and others, coupled to the processor 604 for providing wirelesscommunications for the base station 600. The base station 600 furthercomprises a synchronizer 606 coupled to the processor 604 forcooperating with the processor 604 through well-known techniques to timetransmissions of a uniform power-control bit pattern such that aplurality of base stations, when transmitting, send identicalpower-control bits during each of the plurality of power-control bittimes. The synchronizer 606 preferably is coupled to a conventionalglobal positioning system (GPS) receiver 610 for receiving a timingsignal therefrom. It will be appreciated that, alternatively, anothertype of synchronization system can be utilized instead of GPS, such asLORAN-C, the Network Time Protocol, or other timing systems. It will befurther appreciated that, in some embodiments, the synchronizer 606 canbe incorporated into the processor 604 instead.

In addition, the base station 600 includes a memory element 608 coupledto the processor 604 for storing software instructions for programmingor for execution by the processor 604 in accordance with the presentinvention. The memory element 608 comprises a uniform power-control bitpattern 612 to be sent by different ones of a plurality of base stationsto the mobile unit during a plurality of power-control bit times, beforethe mobile unit is acquired on a reverse link. The memory element 608also includes a synchronization program 614 for programming theprocessor 604 to cooperate with the synchronizer 606 to synchronize thetransmissions. In addition, the memory element 608 includes aconventional communications program 616 for programming the processor604 to control the communications of the base station 600 in accordancewith the protocol employed by the base station 600.

Referring to FIG. 7, a flow diagram 700 of an exemplary method inaccordance with the present invention begins with programming 702 aplurality of base stations with a uniform power-control bit pattern tobe sent to the mobile unit during a plurality of power-control bittimes, before the mobile unit is acquired on a reverse link. Next, thebase stations time 704 their respective transmissions of the uniformpower-control bit pattern such that the plurality of base stations, whentransmitting, send identical power-control bits during each of theplurality of power-control bit times. Timing the transmissionspreferably comprises synchronizing the transmissions through asynchronization signal made available to the plurality of base stations.

In one embodiment, programming the plurality of base stations with theuniform power-control bit pattern comprises programming a pattern thatrequires more than a single transmission frame of a forward link from abase station to the mobile unit before the pattern repeats. In thatembodiment, timing the transmissions comprises synchronizing thetransmissions from each of the plurality of base stations such that thetransmissions start at substantially identical times.

In another embodiment, programming the plurality of base stations withthe uniform power-control bit pattern comprises programming a patternthat repeats after a single one of a plurality of transmission frames ofa forward link from the base station to the mobile unit, and timing thetransmissions comprises starting the transmissions at substantiallyidentical points within different ones of the plurality of transmissionframes.

In yet another embodiment, programming the plurality of base stationswith the uniform power-control bit pattern comprises programming apattern that repeats after a single one of a plurality of transmissionframes of a forward link from the base station to the mobile unit, andtiming the transmissions comprises starting the transmissions atsubstantially identical points within a specific one of the plurality oftransmission frames.

Referring to FIG. 8, an electrical block diagram of an exemplarycommunication system infrastructure 800 in accordance with the presentinvention comprises a central controller 802 for controlling thecommunication system infrastructure 800, and a plurality of the basestations 600 coupled to the central controller 802 for handlingcommunications of the infrastructure 800. It will be appreciated thatthe central controller 802 can also comprise a switch (not shown) forswitching communications present in the infrastructure 800.

Thus, it should be clear from the preceding disclosure that the presentinvention provides a method and apparatus for mitigating power-controlerrors during a soft handoff in a wireless communication system. Themethod and apparatus advantageously is simple to implement in the basestations.

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the precise form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiments were chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled.

1. A method in a wireless communication system for mitigatingpower-control errors during a soft handoff of a mobile unit, the methodcomprising: programming a plurality of base stations with a uniformpower-control bit pattern to be sent to the mobile unit during aplurality of power-control bit times, before the mobile unit is acquiredon a reverse link; and timing transmissions of the uniform power-controlbit pattern such that the plurality of non-co-located base stations,when transmitting for use during the soft handoff of the mobile unit andnot for use during softer handoff of the mobile unit, send identicalpower-control bits during each of the plurality of power-control bittimes.
 2. The method of claim 1, wherein programming the plurality ofbase stations with the uniform power-control bit pattern comprisesprogramming a pattern that requires more than a single transmissionframe of a forward link from a base station to the mobile unit beforethe pattern repeats, and wherein timing the transmissions comprisessynchronizing the transmissions from each of the plurality of basestations such that the transmissions start at substantially identicaltimes.
 3. The method of claim 1, wherein programming the plurality ofbase stations with the uniform power-control bit pattern comprisesprogramming a pattern that repeats after a single one of a plurality oftransmission frames of a forward link from the base station to themobile unit, and wherein timing the transmissions comprises starting thetransmissions at substantially identical points within different ones ofthe plurality of transmission frames.
 4. The method of claim 1, whereinprogramming the plurality of base stations with the uniformpower-control bit pattern comprises programming a pattern that repeatsafter a single one of a plurality of transmission frames of a forwardlink from the base station to the mobile unit, and wherein timing thetransmissions comprises starting the transmissions at substantiallyidentical points within a specific one of the plurality of transmissionframes.
 5. The method of claim 1, wherein timing the transmissionscomprises synchronizing the transmissions through a synchronizationsignal made available to the plurality of base stations.
 6. An apparatusfor use in a wireless communication system for mitigating power-controlerrors during a soft handoff of a mobile unit, the apparatus for usewith a base station of a plurality of base stations attempting tocommunicate with the mobile unit, the apparatus comprising: a processorfor controlling the base station; a memory element coupled to theprocessor for programming the processor, the memory element comprising auniform power-control bit pattern to be sent by different ones of theplurality of base stations to the mobile unit during a plurality ofpower-control bit times, before the mobile unit is acquired on a reverselink; and a synchronizer coupled to the processor for cooperating withthe processor to time transmissions of the uniform power-control bitpattern such that the plurality of non-co-located base stations, whentransmitting for use during the soft handoff of the mobile unit and notfor use during softer handoff of the mobile unit, send identicalpower-control bits during each of the plurality of power-control bittimes.
 7. The apparatus of claim 6, wherein the uniform power-controlbit pattern is a pattern that requires more than a single transmissionframe of a forward link from the base station to the mobile unit beforethe pattern repeats, and wherein the synchronizer is arranged andprogrammed such that the transmissions of the uniform power-control bitpattern from the plurality of base stations start at substantiallyidentical times.
 8. The apparatus of claim 6, wherein the uniformpower-control bit pattern is a pattern that repeats after a single oneof a plurality of transmission frames of a forward link from the basestation to the mobile unit, and wherein the synchronizer is arranged andprogrammed such that the transmissions of the uniform power-control bitpattern from the plurality of base stations start at substantiallyidentical points within different ones of the plurality of transmissionframes.
 9. The apparatus of claim 6, wherein the uniform power-controlbit pattern is a pattern that repeats after a single one of a pluralityof transmission frames of a forward link from the base station to themobile unit, and wherein the synchronizer is arranged and programmedsuch that the transmissions of the uniform power-control bit patternfrom the plurality of base stations start at a substantially identicalpoint within a specific one of the plurality of transmission frames. 10.The apparatus of claim 6, wherein the synchronizer is arranged andprogrammed to synchronize the transmissions through a synchronizationsignal made available to the plurality of base stations.
 11. A basestation for use in a wireless communication system for mitigatingpower-control errors during a soft handoff of a mobile unit, the basestation being one of a plurality of base stations attempting tocommunicate with the mobile unit, the base station comprising: aprocessor for controlling the base station; a wireless transceivercoupled to the processor for providing wireless communications for thebase station; a memory element coupled to the processor for programmingthe processor, the memory element comprising a uniform power-control bitpattern to be sent by different ones of the plurality of base stationsto the mobile unit during a plurality of power-control bit times, beforethe mobile unit is acquired on a reverse link; and a synchronizercoupled to the processor for cooperating with the processor to timetransmissions of the uniform power-control bit pattern such that theplurality of non-co-located base stations, when transmitting for useduring the soft handoff of the mobile unit and not for use during softerhandoff of the mobile station, send identical power-control bits duringeach of the plurality of power-control bit times.
 12. The base stationof claim 11, wherein the uniform power-control bit pattern is a patternthat requires more than a single transmission frame of a forward linkfrom the base station to the mobile unit before the pattern repeats, andwherein the synchronizer is arranged and programmed such that thetransmissions of the uniform power-control bit pattern from theplurality of base stations start at substantially identical times. 13.The base station of claim 11, wherein the uniform power-control bitpattern is a pattern that repeats after a single one of a plurality oftransmission frames of a forward link from the base station to themobile unit, and wherein the synchronizer is arranged and programmedsuch that the transmissions of the uniform power-control bit patternfrom the plurality of base stations start at substantially identicalpoints within different ones of the plurality of transmission frames.14. The base station of claim 11, wherein the uniform power-control bitpattern is a pattern that repeats after a single one of a plurality oftransmission frames of a forward link from the base station to themobile unit, and wherein the synchronizer is arranged and programmedsuch that the transmissions of the uniform power-control bit patternfrom the plurality of base stations start at a substantially identicalpoint within a specific one of the plurality of transmission frames. 15.The base station of claim 11, wherein the synchronizer is arranged andprogrammed to synchronize the transmissions through a synchronizationsignal made available to the plurality of base stations.
 16. A wirelesscommunication system infrastructure for mitigating power-control errorsduring a soft handoff of a mobile unit, the infrastructure comprising: acentral controller for controlling the communication systeminfrastructure; and a plurality of base stations coupled to the centralcontroller for handling communications of the infrastructure, wherein abase station of the plurality of base stations comprises: a processorfor controlling the base station; a wireless transceiver coupled to theprocessor for providing wireless communications for the base station; amemory element coupled to the processor for programming the processor,the memory element comprising a uniform power-control bit pattern to besent by different ones of the plurality of base stations to the mobileunit during a plurality of power-control bit times, before the mobileunit is acquired on a reverse link; and a synchronizer coupled to theprocessor for cooperating with the processor to time transmissions ofthe uniform power-control bit pattern such that the plurality ofnon-co-located base stations, when transmitting for use during the softhandoff of the mobile unit and not for use during softer handoff of themobile unit, send identical power-control bits during each of theplurality of power-control bit times.
 17. The infrastructure of claim16, wherein the uniform power-control bit pattern is a pattern thatrequires more than a single transmission frame of a forward link fromthe base station to the mobile unit before the pattern repeats, andwherein the synchronizer is arranged and programmed such that thetransmissions of the uniform power-control bit pattern from theplurality of base stations start at substantially identical times. 18.The infrastructure of claim 16, wherein the uniform power-control bitpattern is a pattern that repeats after a single one of a plurality oftransmission frames of a forward link from the base station to themobile unit, and wherein the synchronizer is arranged and programmedsuch that the transmissions of the uniform power-control bit patternfrom the plurality of base stations start at substantially identicalpoints within different ones of the plurality of transmission frames.19. The infrastructure of claim 16, wherein the uniform power-controlbit pattern is a pattern that repeats after a single one of a pluralityof transmission frames of a forward link from the base station to themobile unit, and wherein the synchronizer is arranged and programmedsuch that the transmissions of the uniform power-control bit patternfrom the plurality of base stations start at a substantially identicalpoint within a specific one of the plurality of transmission frames. 20.The infrastructure of claim 16, wherein the synchronizer is arranged andprogrammed to synchronize the transmissions through a synchronizationsignal made available to the plurality of base stations.